Comment by Aztek

And not just for tanks.

Comment by Penix2TheMax

All BT epic gem drops also drop inside CoT:Battle for Mount Hyjal now. Also, after a sector of the instance is cleared special mining nodes spawn along the way to the next area which often contain these epic gems.

Comment by Shrinkk

Alternative for this gem is Eye of the Sea which is BoE and can be bought from AH. It's unique-equipped however, so you can have only one.

It can be obtain through fishing daily quest. You also need to be a bit lucky since proc rate is 1% according to wowhead data (but I had it procced 3 times in one month, so I am not sure if 1% info is correct).